Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a precharged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which is modulated (switched on and off, or otherwise controlled, selectively) as it moves, or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively imagewise discharged by the laser in locations to be printed white, to form the desired image on the photoreceptor. The modulation of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon surface; the laser beam from the ROS is reflected by the facets of the polygon, creating a sweeping motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet, as in the well-known process of xerography.
FIG. 1 shows the basic configuration of a scanning system used, for example, in an electrophotographic printer or facsimile machine. Digital data corresponding to the pixels of a desired image to be printed are input in sequence to laser source 10, which is modulated (caused to switch on and off) in response to the stream of digital data. The laser source 10 produces a collimated laser beam 12 which is reflected from the facets 13 of a rotating polygon 14. Each facet 13 of the polygon 14, in turn, deflects the collimated laser beam 12 through an imaging lens 15 to create an illuminated beam spot 16 on the pre-charged surface of photoreceptor 18. The energy of the beam spot 16 on a particular location on the surface of photoreceptor 18, corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, laser source 10 is at the moment of scanning shut off so the location on the surface of photoreceptor 18 corresponding to the pixel will not be discharged by beam 12. It is to be understood that grey levels are imaged in like manner by utilizing exposure levels intermediate between the on and off levels. Thus, digital data input into laser source 10 is rendered line by line as an electrostatic latent image on the photoreceptor 18. Of course, there is a clear need for precise coordination among the flow of imagewise data input to laser source 10, and the motions of the polygon 14 and photoreceptor 18.
In a commercially-practical embodiment of such a scanning system, the resolution of an image on the photoreceptor is typically as high as 600 lines per inch. Therefore, the width of each scan line 20 on the photoreceptor is approximately 42.3 microns. This small scale of the individual scan line 20 necessarily requires very precise tolerances in coordinating the motion of the surface of the photoreceptor 18 and the spot 16 transverse thereto. In a typical imaging system using a servo motor with feedback control for velocity regulation of a drum photoreceptor, low frequency velocity errors are generally removed by the regulating action of the servo system, but because feedback bandwidth is limited, passive damping from the drive motor and drum inertia must be relied upon to suppress higher frequency errors. As a result, residual velocity errors on the order of about 1% can exist at spatial frequencies of around 0.5-2 cycles per millimeter, a scale at which the eye is most sensitive to suble fluctuations. The result on the printed document of this velocity error is referred to as "banding" or "strobing," or "hue rainbows" in a color system.
The actual positional error in a moving photoreceptor associated with these common error sources is typically on the order of a few microns or less, which is much smaller than the cross section of the typical scan line 20. Therefore, correcting for residual motion error by shifting the effective center of spot 16 in compensation need only be by a tiny amount. This shifting of the spot 16 relative to the moving photoreceptor 18 in order to ensure very uniform spacing of raster lines 22 in the process direction is generally known as "beam steering."
One object of the present invention is to facilitate beam steering in response to small positional or velocity anomalies in the movement of the photoreceptor. U.S. Pat. No. 4,428,647 to Sprague et al. describes a multi-beam scanning system wherein each laser of a semiconductor laser array has its own lens mounted adjacent to it, to change the angle of divergence of the light beams so that the light beams may be collected efficiently by an objective lens adjacent the photoreceptor. Although this patent describes a multi-beam system, the invention is concerned with modification of inherent laser source divergence and not beam steering to compensate for motion errors of the photoreceptor.
U.S. Pat. No. 4,514,739 to Johnson et al. discloses a multi-channel ROS including apparatus for detecting errors in the motion of the photoreceptor. These errors are compensated for by means of a plurality of interdigitated addressable drive electrodes. When a predetermined error limit in the position of the photoreceptor is exceeded, an error signal causes the modulation data to be shifted along the electrode array so that the beams strike the photoreceptor at a placement which corrects for the position deviation. Corrections in the position of the beam relative to the photoreceptor can be made only by discrete amounts, limited in precision by the size of the electrodes.
U.S. Pat. No. 5,063,292 to Brueggemann shows an optical system for a ROS scanner wherein the laser from the source is twice reflected off a facet of the polygon, passed through a cylindrical lens to focus the beam in the scan plane, and then reflected off a cylindrical mirror to focus the beam in a cross-scan plane.